In the prior U.S. patent application Ser. No. 287,412 filed on Sept. 8, 1972, a process for the manufacture of alkyl esters of carboxylic acids is described, which consists in reacting a halo-alkane with an alkaline salt of the acid in an heterogeneous organic-aqueous medium, in the presence of a catalytic quantity of a compound chosen from ammonia, primary, secondary and tertiary amines, and the salts of quaternary ammonium, containing altogether at least 10 carbon atoms in the molecule. In these applications, it is recommended to carry on the reaction for a period of time sufficient for the quasi-total reaction of the acid. When the starting acid is polycarboxylic, there is obtained, besides the desired product, intermediate acid esters (named hereinafter partial esters), that is to say, compounds in which the acid residues are not completely esterified. So as to separate these compounds from the reaction mixture, a washing of the organic phase is conducted with an alkaline solution which extracts the acid products, after removal of an aqueous phase containing the alkaline halide. This solution is recycled just as it is to the initial reaction.
When carrying on work in this field, it has been noted that this operating method has drawbacks to the process, for the washing solution is difficult and slow to separate by decantation. Moreover it has been noted that the partial esters recycled to the reaction undergo an hydrolysis.